Our invention relates to automatic controls for effecting ratio changes in a hydrokinetic power transmission mechanism of the kind disclosed in U.S. Pat. No. 4,934,216, which is assigned to the assignee of the present invention. The transmission of the '216 patent includes a hydrokinetic torque converter having an impeller and a turbine situated between an engine crankshaft and a compound planetary gear system, the latter having torque input elements adapted to be clutched to the turbine of the torque converter and output elements adapted to be connected to the traction wheels through a differential and axle mechanism. The transmission of the '216 patent is adapted particularly to be used in a vehicle driveline having a forward engine and rearward traction wheels. It is contemplated, however, that the principles of the present invention can be applied to a transaxle transmission of the type disclosed in U.S. Pat. No. 5,081,886, which also is assigned to the assignee of this invention.
Other examples of prior art transmissions capable of being adapted to incorporate the improvements of our invention are described in U.S. Pat. Nos. 4,978,328 and 5,083,481. These also are assigned to the assignee of this invention.
The transmission of the present invention comprises fluid pressure operated clutch and brake servos that control the relative motion of the elements of the planetary gearing to establish four forward driving ratios, including an overdrive ratio and a single reverse ratio. A hydrokinetic torque converter, which forms a hydrokinetic torque flow path from the engine crankshaft to the input elements of the gearing, includes a turbine and an impeller arranged in a toroidal fluid flow circuit. It includes also a friction bypass clutch adapted to connect the impeller to the turbine to establish a mechanical torque flow path in parallel with respect to the hydrokinetic torque flow path of the converter.
Prior art U.S. Pat. No. 5,029,087 discloses an electronic control strategy for effecting a controlled slip in a torque converter bypass clutch whereby the bypass clutch is actuated by modulated converter clutch solenoid pressure from a clutch solenoid valve to effect varying clutch capacity so that the resulting controlled slip results in an actual slip that approaches a target slip determined by the operating parameters of the driveline. That controlled slip strategy has features that are common to the control strategy for the bypass clutch of the present invention, as will be explained.
In that prior art bypass clutch design, the actual converter slip is continuously monitored by the processor as the engine speed and the turbine speed are detected by engine speed and turbine speed sensors. A desired slip is calculated by the processor during each background control loop and that value is subtracted from the actual slip to detect a slip error. The slip error, in turn, is used to calculate a duty cycle for the clutch solenoid valve so that the error is reduced and the desired slip, together with the actual slip, approach a target value. The magnitude of the target value is a value stored in computer memory. The target slip that is fetched from memory depends upon the operating variables of the driveline. In this manner, the desired slip and the actual slip approach the target value asymptotically.
The bypass clutch strategy of such prior art designs will provide for a continuous slip to eliminate transient torque fluctuations and noise, vibration and harshness during steady-state operation. The non-conformance of the desired slip to the target value due to the asymptotic approach of the desired slip with respect to the target value fetched from memory tends to make calibration of the driveline, particularly the bypass clutch, more difficult. The precise desired slip that is required by the processor to satisfy a particular driveline condition is not available because the desired slip versus time relationship will float: relative to the target value during its asymptotic approach to the target. Furthermore, the continuous slipping that is characteristic of such bypass clutch strategy during steady state operation may cause bypass clutch friction surface durability concerns.